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ORIGINAL RESEARCH article

Front. Mater., 08 July 2020 | https://doi.org/10.3389/fmats.2020.00160

Acceptor and Donor Dopants in Potassium Sodium Niobate Based Ceramics

  • 1Department of Materials Engineering, NED University of Engineering & Technology, Karachi, Pakistan
  • 2Department of Material Science and Engineering, University of Sheffield, Sheffield, United Kingdom
  • 3Department of Physics, Abdul Wali Khan University, Mardan, Pakistan
  • 4The Henry Royce Institute, Sheffield, United Kingdom
  • 5Chemistry Department, King Saud University, Riyadh, Saudi Arabia
  • 6School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal, Malaysia
  • 7Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, China

B-site doping in potassium sodium niobate (KNN) with Mn2+ (MnNb″′) and Ti4+ (TiNb) dopants were soluble but prevented KNN from achieving a high relative density, while Sn4+ (SnNb) was not soluble in the structure as evidenced by second phase peaks in X-ray diffraction (XRD) traces. However, SnO2 was an effective sintering aid in KNN-50/50. A-site doping with Sr2+ (Sr(Na,K)) up to 1 mol% initially improved dielectric properties but higher sintering temperatures were required for compositions with >1 mol% Sr. Samples with 5% and 7% of Sr-doping completely shifted the transition of TO–T to below RT and broadened the TC peaks as the relaxor. All Ti-doped and Sr-doped compositions showed an increase in conductivity, manifested as high values of dielectric loss (tanδ). More than 1% of acceptor and donor dopants showed the ionic-type conduction mechanism, while 1% displayed the electronic mechanism as attributed from the strongly frequency-dependent tanδ. In conclusion, these samples have the potential to open up new applications in the field of electroceramics.

Introduction

Different properties and effects of dopants on potassium sodium niobate (KNN)-based ceramics have been discussed in many publications (Lin et al., 2007; Lee et al., 2008; Wang et al., 2008; Liu et al., 2009, 2012; Tan et al., 2012; Zhao et al., 2013; Bafandeh et al., 2014; Wu J. et al., 2014; Zheng et al., 2015; Yang et al., 2016; Hussain et al., 2019, 2020). Some groups have discussed potassium oxide and sodium oxide both separately and with doping elements, in terms of electrical properties for KNN-based ceramics (Jaffe et al., 1971; Fluckiger and Arend, 1978; Kodaira et al., 1982; Jenko et al., 2005; Lee et al., 2008; Wang et al., 2008; Zhang et al., 2013; Zhao et al., 2013; Zheng et al., 2015). Fewer studies have been done on acceptor (Akça and Yılmaz, 2015; Rafiq et al., 2015; Chen et al., 2016) and donor (Wu S. et al., 2014; Hreščak et al., 2017) dopants in KNN to understand its electrical properties. Donor and acceptor dopants play a vital role in the investigation into the semiconducting properties and defect chemistry of the well-established systems of Barium Titanate (BT) and Lead Zirconate Titanate (PZT; Masó et al., 2006; Da-Wei et al., 2009; Cao et al., 2010; Erdem et al., 2010; Wang D. W. et al., 2011; Wang et al., 2012, 2014; Wang D. et al., 2013; Ma et al., 2012; Zhu et al., 2012; Ali et al., 2013; Freeman et al., 2013; Li et al., 2013, 2015; Lou et al., 2018). Hreščak et al. (2017) reported Sr2+ as a donor dopant in KNN that improved its crystal structure, grain size, and phase composition. Rafiq et al. (2015) reported that the single acceptor-dopant of Mn2+ in KNN decreased the leakage current; most authors, however, have emphasized the importance of co-doping (Guo et al., 2004, 2005; Hollenstein et al., 2005; Hollenstein et al., 2007; Jiang et al., 2007; Lopez-Juarez et al., 2011; Rubio-Marcos et al., 2011; Skidmore and Milne, 2011; Wang H. Q. et al., 2011; Wang K. et al., 2013; Wang et al., 2017; Zhang et al., 2011; Li et al., 2012; Du et al., 2012a, b; Gio and Phong, 2015; Liu et al., 2016, 2017) in achieving optimized functional properties. Chen et al. (2016) prepared different formulations with trace amounts of acceptor-dopants into (K0.5Na0.5 Nb0.994A0.006)O3–δ, where A(Ga3+, Ge4+, Mn2+, Zn2+, Cu2+, and Ni2+) metallic-ions were incorporated; this consequently improved the high mechanical-quality-factor (Qm) values in divalent-doped compositions.

The aim of this research was to introduce acceptor-dopants (Mn2+, Ti4+, and Sn4+) and donor-dopants (Sr2+) into KNN to see the individual effect of their solubility and structural changes on the electrical properties of KNN-50/50-based ceramics.

Experimental Procedure

Nb2O5 was obtained from Stanford Materials Corporation with 99.999% purity, whereas K2CO3 and Na2CO3 were obtained from Fisher Scientific with 99.9% anhydrous used. In addition to this, TiO2 was obtained from Aldrich Chemistry with 99.99%, and MnO2, SnO2, and SrCO3 were obtained from Aldrich with 99.9% purity each. Carbonates were dried at 300°C, whereas all other oxides were heated at 900°C for 24 h. Batches of 50 g of doped-KNN formulations (K0.5Na0.5TixNb1–xO3–x/2 where 0.0 ≤ x ≤ 0.05; K0.5Na0.5MnxNb1–xO3–x/2 where 0.0 ≤ x ≤ 0.01; K0.5Na0.5SnxNb1–xO3–x/2 where 0.0 ≤ x ≤ 0.03; and (K0.5Na0.5)1–xSrxNbO3 where 0.0 ≤ x ≤ 0.07) were prepared from dried raw powders in a hot condition (∼200°C) to avoid the non-stoichiometric conditions caused by moisture, especially in carbonates. All compositions were attrition milled for 1 h in a 500 ml jar at 300 rpm in isopropanol using 3 mm Dia. Y2O3 stabilized zirconia milling media prior to calcination. After milling, the slurry was washed with further isopropanol, separated from milling media through a sieve, and volatiles were removed at 80°C for 24 h in a drying oven. The dried material was sieved through a 150-micron mesh and calcined for 6 h at 850°C at 3°C/min and 5°C/min, heating and cooling rates, respectively. Reacted powders were re-milled before pressing a pellet. 10 mm diameter disks of all compounds were pressed uniaxially with 2-ton force and fired at in a temperature range of 1120–1165°C for 2–8 h. Moreover, the details of sintering temperatures of all formulations are described in Supplementary Figures S1, S2.

Densities of pellets were measured as per the Archimedes method, which were around 90%-bulk in average. XRD traces of sintered pellets were obtained using a Siemens D500 diffractometer at the 2θ range of 10o–80o, using CuKα radiation. The dielectric properties were characterised using an LCR meter (Model 4284A, Hewlett Packard).

Results and Discussion

XRD

The XRD traces from KNN-50/50 doped on the B-site with Mn2+, Ti4+, and Sn4+ are shown in Figures 1, 2. The XRD traces from KNN-50/50 doped with 1% Mn and 1% Ti both appear to be similar, but the shape of 2θ ∼ 45° {220}, and {002} peaks shows a slight difference (Figure 1). In both cases, the intensity of the 2θ = 32° {111} is lower than that of the undoped KNN (Lin et al., 2010). Increasing the concentration of Ti to 5% Ti emphasized the structural change observed at 1% Ti. There are a few studies on single Mn acceptor dopant in KNN. Peaks shifting toward higher 2θ is evidence of shrinkage in the lattice volume, which took place for Mn2+ (Rafiq et al., 2015) and Ti4+ acceptor dopants as compared to the undoped KNN peak (shown in Figure 4; 2θ = 45–46°). Conversely, Lin et al. (2010) reported that XRD peaks shifted toward lower 2θ (i.e., expansion in the lattice volume) in the case of Mn4+ doped KNN.

FIGURE 1
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Figure 1. (A) Single phase XRD patterns of Mn- and Ti-doped KNN-50/50, (B) zoomed peaks at 2θ ∼ 45–46.

FIGURE 2
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Figure 2. (A) XRD patterns of Sn-doped KNN-50/50, (B) zoomed peak at 2θ ∼ 45–46.

For KNN-50/50 doped with Sn4+, secondary peaks of SnO2 are visible in Figure 2. Moreover, within the resolution limits of in-house XRD, there was no discernible change in the trace of the major KNN peaks, confirming that Sn does not enter into solid solution with KNN. Su et al. (2010) investigated SnO2 and CuO co-doping in KNN and concluded that Sn4+ was not soluble after 1 mol%. Their XRD traces also depicted secondary peak positions similar to those in this study. Akça and Yılmaz (2015) also reported insolubility issues for Sn4+ in KNN with secondary peaks visible in their XRD data. The reasons behind the insolubility of Sn4+ issues in KNN is unclear, since Sn4+ (0.69 Å) has a similar ionic radius to Nb5+ (0.68 Å; Shannon, 1976). However, Sn4+ is more covalently bonded to O than Nb5+, which may influence its solubility (Barret, 1962; IBchem, 2016).

For KNN-50/50 doped with Sr2+ on the A-site, the XRD traces revealed a broadening of the {111} peaks as a function of x with respect to undoped KNN-50/50 when fired in both air and N2. Figures 3, 4 should be evaluated together, indicating that Sr2+ was incorporated within the KNN lattice.

FIGURE 3
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Figure 3. (A) X-ray diffraction data of Sr doped KNN-50/50, (B) zoomed peak at 2θ ∼ 45–46.

FIGURE 4
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Figure 4. Zoomed peak at 2θ ∼ 45–46, in comparision with the peak shape of 1% Sr_air/N2 with KNN_50/50_air.

Dielectric Properties

Acceptor Dopants: Mn2+, Ti4+, and Sn4+

The temperature dependence of relative permittivity (εr) and loss (tanδ) of 1 mol% Mn doped KNN-50/50 are shown in Figures 5A,B as a function of temperature and frequency. Two phase transition temperatures, i.e., orthorhombic-tetragonal (TO–T), and tetragonal-cubic (TT–C) are clearly presented, though both transitions are shifted toward lower temperatures compared to undoped KNN-50/50 sintered in air. The εr and tanδ at room temperature of 1% Mn doped KNN-50/50 was 380 and 0.05 at 100 kHz, respectively. However, at lower frequencies, the tanδ increased dramatically when increasing the temperature. The most likely cause of tanδ relates to oxygen vacancy (VO) formed according to the defect equation:

FIGURE 5
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Figure 5. (A) εr and (B) tanδ versus temperature of Mn-doped KNN; (C) εr and (D) tanδ versus temperature of Ti-doped KNN compositions.

Mn Nb ″′ 3 / 2 V O

At high frequencies (1 MHz), tanδ was suppressed with acceptor Mn2+ dopant. At these frequencies, the loss mechanism relating to VO may clamp out, resulting in a decrease in the overall tanδ. There are a number of potential loss mechanisms relating to VO, such as rotation of defect dipoles and movement of space charge. It is not known which mechanism dominates in this study.

The dielectric properties of Ti-doped KNN-50/50 are shown in Figures 5C,D. For low concentrations, (1%) the TT–C temperature decreased with little change in TO–T. However, for high concentrations, TT–C phase transition temperatures increased, and TO–T decreased compared to the undoped and 1% Ti doped compositions and the phase transitions become broader. The peak in permittivity at around room temperature in 3 and 5% Ti-doped KNN may relate to the observation of minor changes to the shape of some XRD peaks (at {220} and {002} planes; Figure 1). Dielectric loss in Ti-doped suppressed from x = 0.01 to x = 0.05 with respect to the temperature and frequency, which suggested that the conduction mechanism shifted from electronic to ionic as the Ti dopant increased. Nevertheless, electronic species are lighter than ionic ones; that’s why ionic species need more activation energy and consequently higher conductions at higher temperatures (see Supplementary Data).

The dielectric properties of Sn4+ doped KNN are shown in Figures 6A,B. Unsurprisingly, there was no effect on the dielectric properties with respect to undoped compositions since there was no evidence that Sn4+ enters the KNN lattice.

FIGURE 6
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Figure 6. (A) εr and (B) tanδ verses temperature of Sn-doped KNN; (C) εr and (D) tanδ verses temperature of Sr-doped KNN compositions.

Donor Dopant: Sr2+

Dielectric properties of Sr donor dopant (A-site) in KNN-50/50 are presented in Figures 6C,D. 1% Sr-doped KNN-50/50 compositions were sintered in both air and N2 to compare the behavior with undoped in a previous study (Hussain et al., 2018) and acceptor doped compositions in the previous sections. However, for undoped KNN sintered in N2, the transition temperature decreased in agreement with 1% Sr-doped KNN (N2). N2 sintered Sr2+ KNN becomes more conductive as compared to undoped KNN, presumably because Sr2+ donates extra electrons (h) and low pO2 creates VO. It has been proposed that the increase in conductivity contributes to the larger permittivity for N2 sintered KNN-1 Sr. TT–C with acceptor (Mn2+) and donor (Sr2+) in air are 390°C and 384°C, respectively, which suggests that disruption of the ferroelectric order to reduce TT–C is greater for A-site Sr2+ doping in comparison to B-site Mn2+. At higher Sr concentrations, x = 0.05 and 0.07, the phase transitions broaden in temperature (inset of Figure 6C), consistent with the broadening of peaks in the XRD traces (Figure 3). In addition, the dielectric loss increased when samples became more conductive at higher than 1% Sr (see Supplementary Data). Nonetheless, this broadening and higher dielectric loss phenomenon with donor doping looks similar to the results reported in the case of W6+ B-site donor doping in KNN, investigated by Wu S. et al. (2014).

Conclusion

Acceptor dopants such as Mn2+ (MnNb″′) and Ti4+ (TiNb) at the B-site of KNN were incorporated to modify properties, but they both inhibited densification of KNN. Nonetheless, both acceptor species were soluble in the lattice, as revealed by XRD. Dielectric losses increased dramatically at lower frequencies with increasing temperatures by using acceptors but were moderately lower at higher frequencies. A further B-site dopant (Sn4+) was also attempted but was insoluble, as evidenced by the appearance of secondary phase peaks in XRD data at low concentrations. Nevertheless, Sn4+ was an effective sintering aid in KNN-50/50 and improved its relative density. Ceramic density improved with 1 mol% Sr2+ but at a higher sintering temperature. Sr2+-doped formulations showed higher conductivity, which manifested itself in higher values of tanδ. TC of KNN-1 Sr in N2 decreased with respect to air-sintered samples.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

FH: Investigation, software, data analysis, original draft writing, and critical discussion of results, etc. AK: Formal discussion, review and editing, and language. ZL: Conceptualization, review, and editing. NA, GW, and DZ: Review, editing, and technical additions. AM: Review, editing, and technical discussion. DW: Review, editing, formal analysis, and critical discussion. All authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

FH acknowledges NED University of Engineering and Technology, Pakistan, for funding support. All authors acknowledge financial help from the Sustainability and Substitution of Functional Materials and Devices EPSRC grant (EP/L017563/1) through Prof. Ian M. Reaney. The data of this work is from Ph.D. thesis of FH (Corresponding Author; Hussain, 2016). ZL acknowledges the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P02470X/1, and EP/P025285/1.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2020.00160/full#supplementary-material

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Keywords: acceptor doping, donor doping, solubility, KNN, dielectric properties

Citation: Hussain F, Khesro A, Lu Z, Alotaibi N, Mohamad AA, Wang G, Wang D and Zhou D (2020) Acceptor and Donor Dopants in Potassium Sodium Niobate Based Ceramics. Front. Mater. 7:160. doi: 10.3389/fmats.2020.00160

Received: 30 March 2020; Accepted: 04 May 2020;
Published: 08 July 2020.

Edited by:

Laijun Liu, Guilin University of Technology, China

Reviewed by:

Zhuo Li, Chang’an University, China
Haibo Yang, Shaanxi University of Science and Technology, China
Yong Li, Inner Mongolia University of Science and Technology, China

Copyright © 2020 Hussain, Khesro, Lu, Alotaibi, Mohamad, Wang, Wang and Zhou. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Fayaz Hussain, fhussain@neduet.edu.pk; engrfayazned@gmail.com; Di Zhou, zhoudi1220@xjtu.edu.cn